This application relates generally to video signals and particularly to progressive and interlaced video signals.
The National Television Systems Committee (NTSC) established a standard for interlaced display of television signals in the United States. In 1941, a monochrome standard (NTSC-I) was established; a color standard (NTSC-II) was established in 1953. The NTSC standard is maintained by the Electronics Industries Alliance (EIA). In Europe, the dominant television standard is known as Phase Alternating Line (PAL). Under both television standards, video signals are displayed by transmitting fields consisting of odd lines and even lines, to the screen. One difference between NTSC and PAL is that NTSC uses a 60 Hz refresh rate while PAL uses a 50 Hz refresh rate.
With this understanding, a typical video camera records video signals as a series of alternating fields: an odd field followed by an even field, for example. One odd field plus one even field constitutes a frame of the video signal. At a frame rate of thirty frames per second, each field is thus captured in successive time periods of {fraction (1/60)}th of a second. Thus, if the odd field of a frame is captured in a first time period, the even field is captured in a second time period, {fraction (1/60)}th of a second later.
So, an image recorded by a video camera may be sent to a television receiver as a video signal. The receiver may send fields to the display, one after another, just as the fields were recorded by the video camera.
A second standard is typically used for the display of an image upon a computer monitor. On most computer monitors, a frame of an image is displayed by starting from the top left of the frame, and, scanning from left to right, displaying each line of the image onto the computer screen, from the first to the last line, until the entire frame of the image has been displayed on the screen.
A progressive scan video camera likewise records each frame of an image by scanning the frame from left to right, for each line, and scanning from top to bottom the entire frame. A camera of this type typically records sixty frames in a second. Recording an image in this manner is well-suited for ultimate display on a computer monitor. The display of progressive video on a personal computer is sometimes called PC graphics.
It is possible for a video signal recorded in an interlaced fashion to be displayed on a personal computer monitor. Likewise, it is possible for a progressively scanned video signal to be displayed on a television screen. In both cases, some conversion of the stored signal such that the signal may be displayed is appropriate.
Environments exist where an interlaced video signal may be combined with a progressively scanned video signal. For example, a receiver may wish to add a progressive signal, such as a graphics overlay, to a video signal, such as a television program. The video signal may be combined with the graphics overlay to produce a new signal suitable for viewing.
To combine the two signals, the odd and even fields of each frame of the interlaced video signal may be xe2x80x9cde-interlaced,xe2x80x9d or combined as a single, progressive, frame. The progressive frame may then be combined with another progressive frame, such as the graphics overlay. Alternatively, a progressive frame may be translated into an interlaced signal and combined with another interlaced video signal.
An interlaced video data stream typically transmits at 60 fields/second. A progressive data stream typically transmits at 60 frames/second, which is twice the rate of the interlaced video data stream.
To combine the data streams in real time, a frame buffer may receive the incoming video data streams such that the streams may be synchronized. For example, a frame buffer which stores three frames of video data, one frame for de-interlacing two fields of video, one frame for buffering the next fields, and one to buffer the graphics, may perform such synchronization.
However, frame buffer memory is not cheap. For example, under the NTSC standard, a single frame of active video data includes 480 lines of 720 pixels/line. If each pixel is described using two bytes of memory, a single stored frame of NTSC video occupies 700 Kbytes of memory. Merely storing two frames of video data for performing the above synchronization, therefore, requires 1.4 Mbytes of memory. Likewise, to store two frames for a PAL system, almost 1.7 Mbytes of memory is used.
If a progressive video signal is interlaced in order to combine with another interlaced video signal, the interlacing may introduce undesirable artifacts into the resulting image. This is particularly true for graphics images, which are higher frequency in nature than video signals which drive an interlaced television. This form of combining is thus inadequate for many real-time applications.
Thus, a need exists for an improved method of synchronizing an interlaced video signal with a progressive video signal without unduly degrading the signals.